Plastic containers having sidewalls with outwardly curled upper rims or lips are well known in the art. During the formation of the upper lip, a significant amount of heat is typically applied adjacent the lip region in order to soften the plastic sidewall material, thereby reducing the amount of force required for curling the lip. However, residual heat remaining in the sidewall after the formation of the lip makes the sidewall difficult to handle for an extended period of time.
Additionally, due to the amount of force required to form the lip, a lubricant, such as mineral oil, is often used for lubricating the interface between the sidewall and a die during the lip curling process. However, excess lubricant can often remain coated on the sidewall, which is undesirable for consumers handling the container.
Further, in cases where the sidewall is constructed of a flat sheet of plastic material that is wrapped to form a cylinder, an overlapping side seam is formed. The overlapping side seam results in a thickened region in the lip that is relatively stiff and requires an increased amount of force during the curling process, as compared to the remainder of the lip. The increased amount of force required in this concentrated region affects the uniformity of the forming process, especially when a die is used to form the lip.
Furthermore, prior art methods and structures often result in a misshapen upper portion of the container. Due to various factors, such as the forces required to form the lip, the stiffness created by the side seam and the memory instilled in the plastic material, the resultant cross-sectional shape of the upper portion of the container is often unpredictable. For example, an oval-shaped cross section may result when a circular-shaped cross section is actually desired.
Accordingly, a need exists for a plastic container having an upper lip that may be formed through a reduced amount of force, heat and lubricant. A need also exists for a blank of plastic material that can be formed into such a plastic container. A further need exists for method of manufacturing such a plastic container.